1. Field of the Invention
This invention relates to methods and systems for modem synchronization in communications systems. More specifically, this invention relates to methods and systems for modem synchronization in which the traditional analog signal is sampled, digitized and reconstructed.
2. Description of Related Art
A variety of modem communications processes and systems have been described in the art. Generally, these prior processes and systems either employ exclusively analog processes or fail to account for and correct the delay induced in the system by the analog-to-digital and digital-to-analog conversions.
The reader is referred to the following U.S. patent documents for general background material. Each of these patents is hereby incorporated by reference in its entirety for the material contained therein.
U.S. Pat. No. 4,059,800 describes a companded digital delta modulator which can encode both voice and voice band modem signals over a wide range of input levels.
U.S. Pat. No. 4,121,217 describes a data transmission network having an improved interface unit for matching the network to local subscriber loops.
U.S. Pat. No. 4,264,974 describes a digital delta modulation compander circuit that makes advantageous use of the mathematical truncation created during computation of step sizes to create a non-linear step size variation which differs from the optimum desired step size but causes convergence recovery during error conditions.
U.S. Pat. No. 4,876,696 describes a transmission system for transmitting modem signals or multifrequency signals together with speech signals over the same digital transmission line.
U.S. Pat. No. 4,910,610 describes a facsimile communication system where four channels of group 3 facsimile signals are transmitted through a digital transmission line of Bearer rate 64 Kbits/s for a speech signal.
U.S. Pat. No. 4,914,689 describes a reverse automatic number identification system, which allows the called party in a telephone line to identify the telephone directory number of the calling party.
U.S. Pat. No. 5,023,906 describes a call progress monitor and algorithm for placing a phone call over the phone lines.
U.S. Pat. No. 5,119,375 describes a system and method for the wireless transmission of information signals between a base station and a plurality of subscriber stations.
U.S. Pat. No. 5,123,033 describes a transmitted modem signal, having an original symbol rate which has been demodulated for digital transmission, that is recovered at a network receiver by obtaining an estimate of error between the original transmitted symbol rate and a known nominal symbol rate.
U.S. Pat. No. 5,127,041 describes a system for connecting a computer to both cellular and landline telephone systems.
U.S. Pat. No. 5,151,923 describes a voice frequency communication apparatus which is capable of detecting and processing the voice frequency terminal signal and non-voice frequency terminal signal and is also capable of realizing communication with ordinary voice frequency terminal signal and non-voice frequency terminal signal without erroneous changeover of the signal processing path.
U.S. Pat. No. 5,175,617 describes a system and method is provided to transmit log map video images through telephone line band-limited analog channels.
U.S. Pat. No. 5,259,000 describes a modem having a modulation and demodulation circuits and a circuit for controlling the modulation and demodulation.
U.S. Pat. No. 5,265,127 describes a non-linear encoder and decoder for information transmission through non-linear channels to mitigate against the multiplicative noise effects caused by known non-linearities in a trellis coded data transmission system.
U.S. Pat. No. 5,291,484 describes a relay and exchange system that includes a plurality of exchanges for relaying and exchanging transmission information contained in frames each composed of m bits, and are so constructed as to form a tandem connection by a plurality of transmission lines.
U.S. Pat. No. 5,305,384 describes an encryption/decryption device, system and method for providing secure communication over a communication channel.
U.S. Pat. No. 5,323,398 describes a speech signal coding system that includes a speech signal encoder for dividing received speech signals into a plurality of frames each having a predetermined time interval and for converting the speech signals into digital speech parameters.
U.S. Pat. No. 5,325,425 describes a call progress monitor and algorithm for placing a telephone call over the telephone lines.
U.S. Pat. No. 5,343,500 describes a non-linear encoder and decoder for information transmission through non-linear channels used to mitigate against the multiplicative noise effects caused by known non-linearities in a trellis coded data transmission system.
U.S. Pat. No. 5,467,353 describes a substrate control channel exchange system that uses a part of bits in a control channel time slot.
U.S. Pat. No. 5,481,567 describes an improvement over existing techniques for transmitting data over voice-band telephone channels by automatically adapting the amount of warping or compression that is applied to a sequence of signal points.
U.S. Pat. No. 5,506,871 describes an adaptive equalizing system that equalizes an unequaled signal and performs carrier recovery for use in a digital communication receiver.
U.S. Pat. No. 5,521,967 describes a call progress monitor and algorithm for placing a telephone call over the telephone lines, that is hierarchically arranged having three major routines or portions: tone detection, signal recognition and situation recognition.
U.S. Pat. No. 5,528,625 describes a quantization-level-sampling modem, which includes a means for separately equalizing each loop of an end-to-end digital telephone system network connection by employing a plurality of transmitter filters and a plurality of receiver filters.
U.S. Pat. Nos. 5,598,401, 5,671,251 describe an apparatus and method for a data communications device to selectively operate in a plurality of analog and digital modes, including as an analog modem, as a digital modem, and as a terminal adapter.
U.S. Pat. No. 5,657,358 describes a toll quality terrestrial wireless digital multiple access terrestrial communication system is disclosed having a base station in communication with telephone lines and with mobile subscriber stations.
U.S. Pat. No. 5,687,194 describes a system for the wireless transmission of multiple information signals utilizing digital time division circuits between a base station and a plurality of subscriber stations.
U.S. Pat. No. 5,734,678 describes a terrestrial RF telephone system that provides a plurality of voice signal channels on a plurality of forward direction carrier frequencies, each carrier frequency having an associated predetermined bandwidth, which is designed to accommodate one analog voice signal, and each carrier frequency containing a plurality of slots.
U.S. Pat. Nos. 5,774,530 and 5,809,120 describe a system and method for using underused or unused local telephone company local loops to subscriber premises for providing to such premises a connection to an alternate service provider.
U.S. Pat. No. 5,815,505 describes a communication device, which is configured to provide selective signal processing at a “plain old telephone service” (POTS) interface, an ISDN U interface, or an ISDN SIT interface.
U.S. Pat. No. 5,842,125 describes a mobile satellite system that includes a satellite communication switching office and network system having a satellite antenna for receiving and transmitting a satellite message via a satellite to and from a mobile earth station including a mobile communication system, a satellite interface system, a central controller receiving and transmitting the satellite message from the mobile earth station to and from the satellite communication switching office via the satellite and the satellite interface system.
U.S. Pat. No. 5,854,835 describes a system and method for using underused or unused local telephone company local loops to subscriber premises for providing to such premises a connection to an alternate service provider having a common channel signaling controlled switched network.
U.S. Pat. No. 5,896,369 describes a mobile communication system and method for connecting a remote workstation to a data network via a mobile communication network.
U.S. Pat. No. 5,926,745 describes a mobile satellite system that includes a satellite communication switching office and network system having a satellite antenna for receiving and transmitting a satellite message via a satellite to and from a mobile earth station.
U.S. Pat. No. 5,999,898 describes a method and apparatus for discriminating between voice and voiceband data in an input signal from a voiceband channel, which is available by blocks of samples.
U.S. Pat. No. 6,002,684 describes a smart subscriber line interface card that includes a switch coupled to a data transmitter, a DAC, and a controller circuit.
U.S. Pat. No. 6,014,374 describes a telecommunication system for processing a plurality of simultaneous bi-directional communications, that includes a primary station and at least one secondary station using wireless transmissions over one of a plurality of available RF carrier frequencies.
U.S. Pat. No. 6,055,245 describes an apparatus and method for facilitating interaction during a communication session between an analog modem and a terminal adapter for conveying digital alert tones and control data there between.